Engineers and safety professionals at plants must work long and hard to eliminate the possibility of fires or explosions in areas where flammable or combustible materials are being processed, handled or stored. This involves identifying all potential ignition sources — whether electrical, hot surfaces, mechanical sparks or naked flames.
However, no matter how well the working environment has been designed, there's one potential source of spark discharges that's ever-present in virtually every workplace and that has enough energy to ignite all common flammable or combustible liquid vapors and gases as well as many airborne dusts and loose solid materials. That energy source is static electricity, also known as "electrostatic" or just simply "static."
Static electricity is the prime culprit for at least two serious fires or explosions in industry worldwide every day of the year, according to the National Fire Protection Association (NFPA) and the U.K.'s Institution of Chemical Engineers (IChemE). In the U.S. alone, static electricity causes on average 280 industrial incidents each year reported to fire and emergency departments, resulting in injuries and fatalities, tens of millions of dollars of direct property damage, lost production or plant downtime, and environmental release issues.
Static electricity is generated continuously through relative motion — in other words, whenever surfaces of materials come into contact and separate. This interaction causes electrons to be stripped from one surface to the other, creating an electrical imbalance. The rate at which electrons are transferred is influenced by a number of factors such as speed and area of contact and the characteristics of the materials (for example, the transfer rate will increase greatly if the materials are dissimilar or one is an insulator). In the workplace typical examples include liquids flowing through pipelines or into drums and tanks, powder dropping down a chute — and even a person walking across an insulating floor. Charge generated in this way often is lost by a combination of conduction to ground and contact with atmospheric moisture (humidity). However, generated charge becomes a serious problem in hazardous areas when it's allowed to accumulate on objects not at ground potential. In these cases, a significant potential (voltage) can develop and, depending on the characteristics of the ungrounded object, this may have many times the surrounding flammable atmosphere's minimum ignition energy (MIE), the minimum energy that can ignite a mixture of a specified flammable material with air or oxygen, measured by a standard procedure.
In any typical working environment hidden dangers may lurk in the hazardous area in the form of "isolated conductors." These are conductive (commonly metal) objects that are either inherently or accidentally insulated from ground. This prevents any static electricity generated from safely discharging, resulting in accumulation of charge on the object. These isolated conductors may exist in commonly used items, including metal flanges, fittings and valves in pipework systems; portable drums, containers and vessels; tanker trucks, rail cars and intermediate bulk containers (IBCs); and even people! Isolated conductors are probably the most likely source of static ignition incidents in industry, ranging from small-scale fires to major damage to plant and injury to personnel.